After computers appeared and applied mathematics is developed, works related to attempts to formalize the medical diagnosis process using mathematical models boomed. The results of these works did not come up to expectations in the beginning and rare successes are connected either with relative simplicity of the problem (to differentiate diseases sufficiently remote from each other in the symptoms space) or with its inadequate simplification. As a result, at best models diagnosing a disease not worse than an average doctor appeared. But after much money and time has been spent for development of a great number of various medical expert systems and objective analysis of their efficiency has been carried out, the accuracy of diagnosis achievable by means of today’s experts systems and expert are very close.